How do I join two SQLite tables in my Android application?

Background

I have an Android project that has a database with two tables: tbl_question and tbl_alternative.

To populate the views with questions and alternatives I am using cursors. There are no problems in getting the data I need until I try to join the two tables.

    Tbl_question  
    -------------
    _id  
    question  
    categoryid  

    Tbl_alternative
    ---------------
    _id 
    questionid 
    categoryid 
    alternative

I want something like the following:

SELECT tbl_question.question, tbl_alternative.alternative where 
categoryid=tbl_alternative.categoryid AND tbl_question._id = 
tbl_alternative.questionid.` 

This is my attempt:

public Cursor getAlternative(long categoryid) {
            String[] columns = new String[] { KEY_Q_ID, KEY_IMAGE, KEY_QUESTION, KEY_ALT, KEY_QID};
             String whereClause = KEY_CATEGORYID + "=" + categoryid +" AND "+ KEY_Q_ID +"="+ KEY_QID;
             Cursor cursor = mDb.query(true, DBTABLE_QUESTION + " INNER JOIN "+ DBTABLE_ALTERNATIVE, columns, whereClause, null, null, null, null, null);
             if (cursor != null) {
                  cursor.moveToFirst();
             }
             return cursor;

I find this way to form queries harder than regular SQL, but have gotten the advice to use this way since it is less error prone.

Question

How do I join two SQLite tables in my application?

You need rawQuery method.

Example:

private final String MY_QUERY = "SELECT * FROM table_a a INNER JOIN table_b b ON a.id=b.other_id WHERE b.property_id=?";

db.rawQuery(MY_QUERY, new String[]{String.valueOf(propertyId)});

Use ? bindings instead of putting values into raw sql query.

An alternate way is to construct a view which is then queried just like a table. In many database managers using a view can result in better performance.

CREATE VIEW xyz SELECT q.question, a.alternative  
   FROM tbl_question AS q, tbl_alternative AS a
  WHERE q.categoryid = a.categoryid 
    AND q._id = a.questionid;

This is from memory so there may be some syntactic issues. http://www.sqlite.org/lang_createview.html

I mention this approach because then you can use SQLiteQueryBuilder with the view as you implied that it was preferred.

In addition to @pawelzieba's answer, which definitely is correct, to join two tables, while you can use an INNER JOIN like this

SELECT * FROM expense INNER JOIN refuel
ON exp_id = expense_id
WHERE refuel_id = 1

via raw query like this -

String rawQuery = "SELECT * FROM " + RefuelTable.TABLE_NAME + " INNER JOIN " + ExpenseTable.TABLE_NAME
        + " ON " + RefuelTable.EXP_ID + " = " + ExpenseTable.ID
        + " WHERE " + RefuelTable.ID + " = " +  id;
Cursor c = db.rawQuery(
        rawQuery,
        null
);

because of SQLite's backward compatible support of the primitive way of querying, we turn that command into this -

SELECT *
FROM expense, refuel
WHERE exp_id = expense_id AND refuel_id = 1

and hence be able to take advanatage of the SQLiteDatabase.query() helper method

Cursor c = db.query(
        RefuelTable.TABLE_NAME + " , " + ExpenseTable.TABLE_NAME,
        Utils.concat(RefuelTable.PROJECTION, ExpenseTable.PROJECTION),
        RefuelTable.EXP_ID + " = " + ExpenseTable.ID + " AND " + RefuelTable.ID + " = " +  id,
        null,
        null,
        null,
        null
);

For a detailed blog post check this http://blog.championswimmer.in/2015/12/doing-a-table-join-in-android-without-using-rawquery

"Ambiguous column" usually means that the same column name appears in at least two tables; the database engine can't tell which one you want. Use full table names or table aliases to remove the ambiguity.

Here's an example I happened to have in my editor. It's from someone else's problem, but should make sense anyway.

select P.* 
from product_has_image P
inner join highest_priority_images H 
        on (H.id_product = P.id_product and H.priority = p.priority)